Influence of granular backfilling material on the segmental lining

TBM drives with precast segmental lining are characterized by an annular void which has to be filled with suitable material. In the process of tunnelling in hard rock, the annular gap is usually filled with fine grained and closely graded gravel, termed as pea gravel. The backfilling process is conducted as soon as possible after each regripping process. The relevant criterion to provide sufficient bedding is the stiffness of the annular void filling material.

Due to the incomplete bedding situation cracking within the segmental lining occurs. This only indicates an increased load factor, but not a threat to the rate of utilization.

Mechanical treatment of fault material

Although fault zones are encountered in most cases when excavating underground structures, the mechanical properties of the material such fault zones are composed of are still hard to determine. At least in a way that the ground and system behaviour can be predicted reliably without extensive at site investigations and numerical simulations. Why? Because of

the heterogeneous and anisotropic structure of fault zones;

the countless variety of possible formations of such weak rock mass zones;

the particular primary stress state due to recent and prehistoric tectonic movement;

the stiffness and strength contrast between the fine-grained, strongly sheared matrix and intact rock block inclusions;

the different shapes and sizes intact rock block inclusions can form in relation to the underground structure;

the difficulties to test undisturbed fault material samples with common laboratory apparatus;

The research Alexander Kluckner is going to do comprehends amongst others following tasks:

Investigate analytical approaches for the determination of the mechanical properties of fault material under consideration of the classical elasticity theory.

Extensive evaluation of case studies focusing on topics like 'the occurrence of relatively large blocks within a fault zone, which led to major problems', 'the design approach for structures in fault zones prior to excavation and its success', 'the measures executed, as an unpredicted fault zone has been encountered' etc.

Extensive field mapping program to investigate the variety of fault zone formations.

Physical modelling of tunnels in hard rock

When analysing complex rock mass structures composed of hard rock intersected by zones of faulted and sheared material, anisotropic rock mass or cohesionless material with continuous numerical calculations all possible behavior types cannot be investigated in a proper way. In order to circumvent the shortcomings of continuous simulations and to determine the relevant factors influencing the tunnel deformation behavior and the failure mechanisms laboratory tests on artificial rock mass models can be conducted.

In the current research a test apparatus has been used to be able to investigate hard rock conditions. Based on a homogenous model, features like faults, fault zones, slickensides and lamination can be assembled additionally. The forces acting on the model are induced by one vertical and one horizontal cylinder. Therefore a three dimensional stress state and a plain strain state is generated. The behavior of the excavation and the failure mechanisms around it in response to the continuously increasing stress level can be observed through an acrylic glass front plate.

In order to analyze the ground behavior, the overall stress level applied on the external cylinders, the deformation behavior determined by using the particle image velocimetry and the evaluation of failure mechanisms are used. Therefore insitu conditions of rock masses can be modeled in order to get a better understanding of the actual deformation behavior.

Remote Characterization of Rock Masses

Discontinuities dominantly influence the mechanical behaviour of rock masses. Thus, it is of crucial importance in rock mechanics to have a profound knowledge about the discontinuity network. Traditional measuring techniques provide a rough knowledge about a discontinuity network but are also limited within reachability, geological/geotechnical knowledge, time and scale. The results are subjective and not reproductive.

The application of remote sensing techniques like photogrammetry and laser-scanning, in geotechnics helped to collect data about the discontinuity network and reduce the bias, which is introduced by manual mapping. Together with numerical models, reliable predictions about the stability of the rock face and the discontinuous rock are nowadays possible. But there is still missing an automatic data collection, as well as an automatic implementation of the collected data in a numerical rock model.

In his research, Andreas Buyer focusses on the automatic mapping of discontinuities aided by remote sensing techniques using ShapeMetriX3D (3GSM GmbH). This means:

Extraction of discontinuity features from digital images and digital surface models;

Students, who are interested in the research topics described are very welcome to contribute to the research progress by writing a scientific thesis. Engineers around the world are invited to share thoughts and experiences in order to bundle strengths.